1. Field of the Invention
The present invention relates in general to a novel numerical optical processer system which utilizes a holographic content-addressable memory. The holographic memory stores relationships between various inputs and system outputs in the form of a plurality of truth tables in a digital format. One or more digital input words are processed by the disclosed numerical optical processing system to produce digital outputs wherein the one or more digital input words are modified by the contents of the holographic memory.
2. Description of the Prior Art
There is a growing class of processing problems which require the very high system throughputs that only parallel processing can provide. Such problems include remote sensing, automatic inspection, air traffic control, defense early-warning systems, automatic surveillance, meteorology, and coordinate position locating. Electronic digital techniques have demonstrated the required accuracy, speed, and flexibility to solve an enormous variety of processing problems. However, attempts to extend these techniques to parallel processing have proven to be very expensive and have introduced reliability problems. Because of the inherently parallel nature of optical processing systems, they are well suited for these problems. Optical processing systems of the past have been primarily analog in nature and have therefore sacrificed some of the accuracy and flexibility that digital techniques can provide. There have been, therefore, continuing efforts to combine optical techniques and digital numerical methods to obtain the benefits of both.
Previous optical data processing systems fall into two categories: Those that perform rudimentary logic operations on inputs and those that perform a specific numerical operation, or at best a limited range of operations, on the inputs. Many of these systems require unusual or awkward forms of input and output signals. The present invention is capable of implementing any combinational logic function or binary numerical operation which can be represented by means of a truth table. Inputs and outputs to the system are in the form of convenient binary-value electrical signals.
In a binary processing system, a truth table defines the relationship between the inputs and the output. The output values are tabulated for all possible combinations of the input values. Any combinational logic function or numerical operation can be represented by such a truth table. Among the numerical operations possible are: addition, subtraction, multiplication, division, trigonometric functions, exponentiation, and general polynomial functions of one or more variables. In a truth table look-up processer, each bit in the output answer is determined by comparing the input words with all combinations in the truth table which cause the particular output bit to be a logical "1". In operation, these systems compare the given input data pattern to the stored patterns associated with each output bit in the answer. Output bits corresponding to tables in which a matching pattern is found are set to a logical "1". If no match is found within a table, the associated output bit is set to logical "0".
Few numerical processing systems have been constructed using truth table look-up methods because of the large memory capacity required to store a table of reference patterns for each output bit. The large holographic storage capacity of thick recording materials such as electro-optic crystals can provide the necessary memory size. Electro-optic crystals possess the dynamic range necessary to record the large number of phase holograms required, as well as the stability to maintain the needed precise phase relationships of the reconstructed wavefronts. The thickness of electro-optic crystals permit the angular multiplexing of holograms which is necessary in some circumstances.
Designs for optical holographic location-addressable memories and for some forms of optical holographic content-addressable type memories are not suitable for truth table look-up processing of the type disclosed by the present application. Prior art optical holographic content-addressable memories are of the wavefront auto-correlation and cross-correlation type. These memories suffer from output light intensities which are essentially analog in nature and must therefore be threshold detected to produce a binary output. The present invention includes forms of optical holographic content-addressable memories which are not of the correlation type and thus produce outputs that are essentially discrete in nature, thereby lending themselves to parallel digital processers of the type described herein.